1. Field of the Invention
The invention relates generally to the field of earth penetrating and boring particularly by using high pressure water jets.
2. Description of the Prior Art
Conventional drilling and mining techniques using mechanical equipment are well known in the art. High pressure water jets for fluid erosion in mining and drilling are also well known. Examples of such art are the patents to Summers, et al U.S. Pat. No. 4,119,160 and U.S. Pat. No. 4,106,577. The continuing and increasing demand for energy has dictated that new techniques be devised for increasing the available supplies of energy, and a conversion into convenient and distributable forms. Some of these techniques currently are directed to the in situ gasification of coal to produce a combustible gas as a replacement for natural gas. These techniques also include new means for extracting oil from existing wells where the oil is bound up in very viscous mixtures such as tar sand or oil shale.
One technique that has been used in the in situ gasification of coal has been the employment of vertical wells spaced approximately 100' apart and linked by a horizontal hole in the coal seam. The horizontal link is accomplished by reverse combustion burning and directional drilling. In the case of reverse combustion, the fire is propagated from the base of one vertical well to the other by forcing air down one well while the fire is initiated at the other. The air flows to the flame front through the bedding planes and hopefully burns back toward the air supply. This method has been successful only about half of the times tried with the major reason for failure being that the fire overrides the coal seam.
Tests have also been made using directional drilling to achieve linkage between adjacent wells. These techniques are similar to directional drilling employed in drilling oil wells. Most generally the minimum radius for such drilling is about 100'. In addition, maintaining the alignment or elevation of the drills so as to stay within the coal seam is difficult to achieve. Moreover, when long distances are involved, the frictional forces become great and unless the thrust is controlled within a particular range, the drill bit can actually travel above or below the seam. The problems encountered in the directional drilling using conventional means are described in considerable detail in the paper entitled "Directional Controlled Drilling to Horizontally Intercept Selected Strata, Upper Freeport Coal Bed, Green County, Pa." by William P. Diamond, David C. Oyler and Herbert H. Fields. This report is published by the U.S. Department of the Interior, Report of Investigations No. 8231. In one test reported, it took 41/2 months to drill a 200' horizontal hole at a depth of 1000'.